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Ammonia modeling for assessing toxicity to fish species in the Rio Grande, 1989-2002 Howard D. Passell Sandia National Laboratories Geosciences and Environment Center Clifford N. Dahm University of New Mexico Dept. of Biology Edward J. Bedrick University of New Mexico Dept. of Math and Statistics

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Addressing uncertainty in the data Besides NH 4 +, pH data are most critical After various sensitivity analyses, we made  60 sets of SSWRP pH data, using mean and SD from existing data  60 sets of Rio Grande pH data using different means and different SDs for different years We ran the model 60 times, with a different set of SSWRP pH data and a different set of Rio Grande pH data in each run, and then aggregated the results

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Model assumptions -- 3 No mixing of toxicants  Buhl (2002) tested toxicity to silvery minnow and fathead minnow in a mix that simulated the water of the Rio Grande Chlorine in the MRG was most toxic – 96-hr LC50 0.114 mg/L Copper was second – 96-hr LC50 0.250 mg/L NH 3 was third – 96-hr LC50 1.0 mg/L  Copper and NH 3 accounted for 93-98% percent of toxicity, and toxicity was more than additive  Chronic criterion could be as low as 0.001 mg/L N-NH 3, based on the mix  Site specific acute and chronic criteria might be appropriate for the Rio Grande

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Rio Grande silvery minnow, once one of the most common fish in the Rio Grande is now limited to about 5% of its former range, between Cochiti Dam and Elephant Butte Dam Four other cyprinids were made extinct or extirpated from the Rio Grande (3/4 in the last 40 years)  Extirpated Rio Grande shiner (Notropis jemezanus) last collected between 1901 and 1950 Speckled chub (Extrarius aestivalis), last collected in 1960s  Extinct Phantom shiner (Notropis orca), last collected in 1964 Bluntnose shiner (Notropis simus simus), last collected in 1975 EPA acute criteria for N. spilopterus and N. whipplei are less than ½ the acute criterion for the fathead minnow  Notropis may be a genus generally more sensitive to NH 3 What is the relevant ecological history of Rio Grande fish community?

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NH 3 concentrations may have been high for decades prior to 1989 NH 3 contributed to the extirpation and extinction of native cyprinids NH 3 contributed to the current endangerment of the silvery minnow (along with other causes) Improvements at the SSWRP resulted in 2002 average NH 3 -N concentrations of 0.0004 mg/L, but N H 3 could still pose a threat with: Increasing populations upstream and downstream of Albuquerque Accidental spills Synergistic effects of mixed toxicants Declining water quality is a hidden consequence of drought in effluent-influenced streams Conclusions

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NH 3 toxicity may have created a barrier to silvery minnow migration in the Rio Grande. Success of current plans to create minnow refugia upstream of Albuquerque may be enhanced by the removal of that barrier NH 3 toxicity could be playing a large role in rivers around the world, especially in developing nations where sewage treatment is limited or absent. Conclusions -- 2